266 THE EAR
disrupted bone conduction will be better heard than air conduction and
Rhine's test is said to be negative. In^PJiductiye^eafness Rinne^s test will be
negative if the conductive hearing loss is greater than 15 decibels.^
Absolute Bone Conduction
In tuning fork bone-conduction tests the normal sound-conducting pathway
is by-passed, so that bone-conduction tests are a meas^e of inner ear function.
In the .test .of. "absolute bone conduction, the bone conduction of the"patient
isjjpmoared with^hat.pf tEe 'exammeT and;;assuming "that ~the exafriiner fcas-
nprjoaaliearipg, it is^a test of the patient's inner ear functions,. The tuning
fork is struck and its base is applied to the patrentTi^idjjrocess wit
meatus occluded. The patient signals as soon as the sound of the fork is no
longer heard. The tuning fork is immediately applied to the examiner's
mastoid process with the meatus occluded. If the examiner can still hear the
tuning fork the patient's inner ear function is reduced and bone conduction
is said to be shortened. In conductive deafness, and particularly in otosclerosis,
bone conduction may be lengthened, i.e. the patient continues to hear the
tuning fork when the examiner can no longer hear it.
This tests bone conduction and..is.jof greatest yaluejrjLcasej,.Qf jasyjooupietncal
'conductive i deafness. The tuning fork is set in vibration and applied to the
vertex of the skull in the midline and the patient is asked in which ear he
hears the sound. The vibrations are transmitted equally to the cochlea on
each side. In sensorineural deafness one would expect the sound to be better
heard in the better ear. While this sometimes occurs it is not a regular finding,
the sound being more often heard in the midline. |n conductive deafness, sm.
tbgjjjj^han.^ the Lsouncl. Jshe^d hithe mare.af^ted eaj.
Provided that the patient's reply is accurate Weber's test is very sensitive
in conductive deafness. In unilateral conductive deafness, Weber's test will
be heard in the affected ear where the hearing loss is only 5 dB. The student
can verify this by applying Weber's test to himself and producing a mild
conductive deafness by occluding the external acoustic meatus on one side.
The sound of the tuning fork will be clearly heard in the occluded ear.
False Negative Rinne
This is an important observation. In unilateral total or severe sensorineural
deafness Rinne's test will appear to give a negative result. Air conduction is
absent, but bone conduction may be good because the sound is transmitted
to the opposite cochlea. This result may lure the examiner into making a
wrong diagnosis of conductive deafness. In this situation Weber's test is
important. If the deafness is of conductive type, the tuning fork should be
heard in the deaf ear. If Rinne's test is negative in one ear and Weber's test
is not lateralized to that ear the examiner must consider that the condition
may be total deafness. The final test is a loud shout in this ear with Bardny's
noise box in the opposite ear.
A pure-tone audiometer is an instrument which delivers sound of set
frequency to the ear by earphones. The frequencies usually tested are at